Method and apparatus for detecting water in a fluid media

Abstract

An apparatus and method for measuring the concentrations of oil, water and / or hydrates in a multiphase production fluid flow comprises measuring the complex permitivities, both real and imaginary parts, of the flow at at least two frequencies. One dispersive and one non-dispersive frequency are used. In one embodiment at least three frequencies may be selected, one of which is below about 1 MHz while the other two are within the dispersive and non-dispersive ranges for water. In this embodiment water may be distinguished from hydrates and both measured simultaneously.

Description

BACKGROUND [0001] 1. Field of Invention [0002] This invention relates to the field of methods of measuring water and hydrates content in production fluids. More particularly, it relates to methods and apparatuses for measuring water in multiphase production fluid flows. [0003] 2. Background Art [0004] Many fluids, such as hydrocarbon production fluids, contain a certain proportion of water. Generally this water is in one of two forms, either as liquid water, or hydrocarbons hydrates. These hydrates are ice-like minerals that form at moderately low temperatures and high pressures of the deep sea and some land applications. They contain a molecule of hydrocarbons to which 4 or more molecule of water bond themselves in a symmetrical cages to form hydrate crystals. Regardless of form, however, the presence of water along with the more desired components of the production fluid, such as oil, is often problematic. If in liquid form, the water must eventually be removed in order to enable ...

AI Technical Summary

Benefits of technology

[0010] This invention teaches a method and the apparatus for measuring water cut in crude oil which does not depend on differences in dry oil properties: density, viscosity, permittivity, conductivity, etc. hence it is simple, reliable and inexpensive. From the prior art one can see that there is need for accurate, reliable, simple apparatus to detect and measure natural hydrates in multiphase flow in the pipeline, in tank or in any other vessel,

Problems solved by technology

Regardless of form, however, the presence of water along with the more desired components of the production fluid, such as oil, is often problematic.

If in hydrate form, the water plays a part in plugging pipelines and presenting transportation and storage difficulties.

However, many of today's methods of determining the proportion of the so-called “water cut”, i.e., the proportion of the production fluid which is water in either liquid or hydrate form, are inaccurate as each hydrocarbon has different permittivity and or density.

So unless individual calibration is performed, large error levels may result that can be as high as 5 percent.

Current water cut meters are particularly susceptible to this high error level.

This inaccuracy is often unacceptable.

This method, unfortunately, is subject to systematic error.

This systematic error results from the differences in the complex permittivity of different crude oils.

Prior art methods have, however, failed to fully acknowledge this complexity of composition by using only overall density in its determination of complex permittivity.

However, because it is possible for two different dry crude oils, of different compositions, to have the same density but different dielectric constants, the dielectric constant is likely to be incorrect for the composition as a whole.

Thus, even extremely accurate measurements of crude oil density may result in a crude oil dielectric constant with an error equivalent to 0.7 to 1.0 percent of water cut value.

This degree of error is considered to be very high in many applications.

Importantly, this method is also generally unsuited to in-line use in multiphase flows of production fluid.

The drawback to this indirect method is that chemically active additives, such as methanol, salt, glycol and some other substances, substantially affect these calculations.

Use of calculations rather than actual measurement, however, again leaves significant room for error.

Generally speaking, the focus of attention of such research centered on the properties of hydrates rather than devising practical instruments for detecting the formation of hydrates and measuring their concentration, without prior knowledge of the properties of the hydrocarbons and salinity of the water in the conduit.

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